I n situ ellipsometry experiments have been used to study the modification of hydrogenated amorphous silicon (a-Si:H) surfaces exposed to He and Ar plasmas at room temperature in an asymmetric capacitively coupled radio-frequency (rf) glow discharge reactor. Such a system provides an informative way of studying the microstructural changes caused by ion bombardment at two different accelerating potentials. A photon energy of 3.5 eV is used as a probe in the ellipsometry experiment in order to maximize sensitivity to changes in the volume density of Si–Si bonds near the sample surface. As expected, significant differences are observed between the effects of exposure at the anode and the cathode owing to the greater potential difference between the cathode and the plasma. When the a-Si:H is exposed to a 0.3 W/cm2 He plasma at the cathode (∼–30 V dc bias), a surface layer with a low density of Si–Si bonds is formed, reaching a thickness of ∼35 Å after 30 min. For a fixed time interval and plasma power, the thickness of the low density layer caused by the He plasma is about 2–3 times greater than that caused by Ar. The surface layer is readily consumed by oxidation and its properties are attributed to the presence of microvoids ≲50 Å in size. For samples exposed to a 0.3 W/cm2 He plasma at the anode (grounded), no significant damage is detected. In this case the effect of the plasma is to remove a very thin oxide overlayer. Thus, ellipsometry experiments are effective in determining plasma operating conditions which result in improved surfaces while minimizing roughness layers and damage.